The present invention relates to an improved method and apparatus for preparing low-concentration polysilicate microgels, i.e., aqueous solutions having an active silica concentration of generally less than about 1.0 wt. %, which are formed by the partial gelation of an alkali metal silicate or a polysilicate, such as sodium polysilicate, having in its most common form one pan Na.sub.2 O to 3.3 parts SiO.sub.2 by weight. The microgels, which are referred to as "active" silica in contrast to commercial colloidal silica, comprise solutions of from 1 to 2 nm diameter linked silica particles which have a surface area of at least about 1000 m.sup.2 /g. The particles are linked together during preparation, i.e., during partial gelation, to form aggregates which are arranged into three-dimensional networks and chains. A critical aspect of the invention is the ability to produce the microgels within a reasonable time period, i.e., not longer than about 15 minutes until the microgel is ready for use, without the risk of solidification and with minimum formation of undesirable silica deposits within the processing equipment. Polysilicate microgels produced according to the invention are particularly useful in combinations with water soluble cationic polymers as a drainage and retention aid in papermaking. At low pH values, below pH of 5, these products are more appropriately referred to as polysilicic acid microgels. As the pH value is raised, these products can contain mixtures of polysilicic acid and polysilicate microgels; the ratio being pH-dependent. For sake of convenience, these products hereinafter will be referred to as polysilicate microgels.
The present invention is an improved method and apparatus for continuously preparing a low-concentration polysilicate microgel which comprises: (a) simultaneously introducing a first stream comprising a water soluble silicate solution and a second stream comprising a strong acid having a pKa less than 6 into a mixing zone where the streams converge at an angle of not less than 30 deg and at a rate sufficient to produce a Reynolds number of at least about 4000 and a resulting silicate/acid mixture having a silica concentration in the range of from about 1.0 to 6.0 wt. % and a pH in the range of from 2 to 10.5; (b) aging the silicate/acid mixture for a period of time sufficient to achieve a desired level of partial gelation (i.e., forming the microgel), usually for at least 10 seconds but not more than about 15 minutes; and (c) diluting the aged mixture to a silica concentration of not greater than about 1.0 wt. % whereby gelation is stabilized.
For best results, the silica concentration of the water soluble silicate starting solution is in the range of from 2 to 10 wt. % silica, and the concentration of the strong acid (e.g., sulfuric acid) is in the range of from 1 to 20 wt. % acid as the two streams are being introduced into the mixing zone. The preferred conditions in the mixing zone are a Reynolds number greater than 6000, a silica concentration in the range of 1.5 to 3.5 wt. % and a pH in the range of 7 to 10. The most preferred conditions are a Reynolds number greater than 6000, silica concentration of 2 wt. % and a pH of 9.
The apparatus according to the invention comprises: (a) a first reservoir for containing a water soluble silicate solution; b) a second reservoir for containing a strong acid having a pKa of less than 6; (c) a mixing device having a first inlet which communicates with said first reservoir, a second inlet arranged at an angle of at least 30 deg with respect to said first inlet which communicates with said second reservoir, and an exit; (d) a first pumping means located between said first reservoir and said mixing device for pumping a stream of silicate solution from said first reservoir into said first inlet, and first control means for controlling the concentration of silica in said silicate solution while said solution is being pumped such that the silica concentration in the exit solution from the mixing device is in the range of 1 to 6 wt. %; (e) a second pumping means located between said second reservoir and said mixing device for pumping a stream of acid from said second reservoir into said second inlet at a rate relative to the rate of said first pumping means sufficient to produce a Reynolds number within said mixing device of at least 4000 in the region where the streams converge whereby said silicate and said acid are thoroughly mixed; (f) mixture control means located within said exit and responsive to the flow rate of said acid into said mixing device for controlling the pH of the silicate/acid mixture in the range of from 2 to 10.5; (g) a receiving tank; (h) an elongated transfer loop which communicates with the exit of said mixing device and said receiving tank for transferring said mixture therebetween; and (i) a dilution means for diluting the silicate/acid mixture in the receiving tank to a silica concentration of not more than 1.0 wt. %.
In an alternate embodiment, the apparatus of the invention includes a NaOH reservoir and means for periodically flushing the production system with warm NaOH which has been heated to a temperature of from 40.degree. to 60.degree. C. whereby deposits of silica can be solubilized and removed.
In a further embodiment of the invention, an agitating gas stream such as a stream of air or nitrogen or other inert gas can be introduced into the mixing device described by means of an additional inlet located at or near the mixing junction. Gas agitation provides an important industrial benefit in that it permits low silicate flow rates to be employed while maintaining the required turbulence and Reynolds number in the mixing zone.
In yet a further embodiment of this invention, mixing of the acid and the water soluble silicate solution can be accomplished in an annular mixing device. This device can be an internal pipe or tube which protrudes into and subsequently discharges inside of a larger pipe or tube. The internal pipe discharge point is usually, but not necessarily, concentrically located inside the external pipe. One of the two fluids to be mixed is fed into the internal pipe. The second fluid is fed into the external pipe and flows around the outside of the internal pipe. Mixing of the two fluids occurs where the first fluid exits the internal pipe and combines with the second fluid in the larger external pipe.
For the purpose of mixing the two liquids, the water soluble silicate solution and the acid can be fed to either the internal or the external pipes at rates sufficient such that when the two streams are combined, a Reynolds number of greater than 4000 is produced in the mixing zone. An agitating gas stream can also be optionally employed to aid in the mixing of the two streams.
As a further embodiment to this invention, mixing of the acid and water soluble silicate solution can be accomplished in a vessel equipped with mechanical means to create the necessary turbulence, such that mixing of the two streams is accomplished at a Reynolds number of greater than 4000. The vessel can optionally be equipped with baffles. The acid and water soluble silicate solution can be but do not have to be fed to the vessel simultaneously.
The method and apparatus of the invention are capable of producing stable polysilicate microgels resulting in reduced silica deposition within a convenient time frame of not more than about 15-16 minutes, but usually within 30 to 90 seconds, without the risk of solidification and with minimum formation of undesirable silica deposits within the processing equipment. Temperature of operation is usually within the range of 0.degree.-50.degree. C.
Silica deposition in production apparatus is undesirable because it coats all internal surfaces of the apparatus and can impede the functioning of vital moving parts and instrumentation. For example, silica deposition can build to the point where valves can no longer function and can restrict fluid flow through pipes and tubing. Deposition of silica is also undesirable on the pH sensing electrode as it prevents monitoring the process pH, a critical quality control parameter for silica microgel production.